Automotive fluids, such as engine oil or transmission fluids, absorb heat in use. To prevent fluid deterioration, this heat often needs to be removed. Heat exchangers are commonly used for this purpose. Moreover, heat exchangers are known to perform this function adequately in moderate ambient conditions. However, in cold ambient conditions, engine oils and transmission fluids can be highly viscous. In such conditions, automotive fluids do not flow easily through heat exchangers. As a result, in such conditions, the interposition of a heat exchanger in an oil circuit can disadvantageously impede circulation. Starvation of some downstream components, like transmissions, may even occur.
In order to avoid these adverse effects, it is known to provide a mechanism for bypassing the heat exchanger. One way that this has been done in the past is to provide a bypass conduit. The bypass conduit is connected in parallel with the heat exchanger and has a relatively low resistance to the flow of high viscosity fluids as compared to the heat exchanger. Structures of this type are known to avoid starvation of downstream components, but can suffer in that, in normal operating conditions, the flow is split between the heat exchanger and the bypass circuit. This requires that the heat exchangers be made proportionately larger and heavier to achieve the same overall heat exchange performance for the cooling system. This added size and weight, and the added costs associated therewith, are undesirable to automotive manufacturers.
To ameliorate the split-flow problem, it is known in the prior art to provide bypass valves. Sometimes, these bypass valves are pressure-activated, and are integrally constructed with or attached to the heat exchanger. A structure exemplary of the foregoing is shown in U.S. Pat. No. 5,236,043 (Armbruster), issued Aug. 17, 1993. This structure includes a flapper valve of spring steel biased in a closed position, to arrest bypass flow, and which is adapted to be urged open when the flow resistance through the normal passage of the heat exchanger is too high as in of cold-start conditions. Heat exchangers of this general type can avoid starvation of downstream lubricated components, and can be adapted such that bypass flow is substantially nil in normal operating conditions, thereby to permit compact heat exchanger construction. However, in Armbruster, connection of the flapper valve to the heat exchanger body is effected by a press-fitted stud. Such construction is difficult to accomplish and suffers from a propensity to leak.
Another type of flapper valve is shown in U.S. Pat. No. 3,998,571 (Falke), issued Dec. 21, 1976, wherein a flapper valve for the cylinder of a reciprocating compressor is shown. This flapper valve is part of a flapper sub-assembly having a flapper mounting portion riveted in place. However, the rivet construction also has a propensity for leakage, and the riveted sub-assembly requires separate handling and increases the cost and complexity of the device.